The present invention relates to a medium for optically recording information and, more particularly, to an optical recording medium which receives an energy beam such as a laser beam of an intensity varying in accordance with information and which records information by melt-deforming in accordance with the intensity.
Conventionally, an optical recording medium such as an optical disc comprises a substrate and a recording layer which is formed on the substrate. When information is recorded on the optical disc, an energy beam such as a laser beam which changes its intensity in accordance with the information is radiated on the recording layer. The recording layer melt-deforms in response to the laser beam. As a result, a pit array corresponding to the information is formed on the recording layer.
A material comprising the recording layer is generally required to satisfy the following conditions. In the first place, the material must have good sensitivity (initial characteristic), in other words, the material must require only a small amount of energy (mJ/cm.sup.2) per unit area for forming the pit. In the second place, the material ideally must not change over time or it may change slightly. The material must have a long service life. Currently, tellurium (Te) is used as the material for the recording layer. Te has good sensitivity for an energy beam such as a laser beam, so that tellurium is one of the most favorable materials in this field.
However, when Te is exposed to the outer atmosphere, Te is easily oxidized by oxygen or moisture which is included in the air. An undesirable oxide tends to be formed on the surface of the layer. As a result, tellurium changes over time, thus degrading its sensitivity. In other words, Te satisfies the first condition. However, Te fails to satisfy the second condition. For example, when Te is exposed to an atmosphere under severe testing conditions such as a temperature of 70.degree. C. and a relative humidity of 85%, the sensitivity of the Te layer degrades by about 20% in about 5 hours. The sensitivity degrades by about 50% in about 15 hours. The service life characteristic obtained in the above test indicates that information recording with stability and high quality is not possible with a conventional recording medium comprising a Te layer as the recording layer. On the other hand, in order to improve the anti-oxidant characteristics of the Te layer, a protection layer comprising, for example, an organic material is coated on the Te layer. In this manner, a method for sealing the Te layer from the outer atmosphere has been performed by formation of a protection layer on the Te layer. However, the capacity of the protection layer which seals the Te layer from the outer atmosphere is low because the organic material gradually absorbs moisture as time passes. As a result, a desired and adequate service life, that is, at least 10 years in the normal atmosphere (normal temperature and humidity), has not been accomplished.